LibWeb: Move DOM classes into the Web::DOM namespace

LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.

Libraries/LibWeb/CSS/StyleSheetList.cpp

@@ -33,7 +33,7 @@ void StyleSheetList::add_sheet(NonnullRefPtr<StyleSheet> sheet)
     m_sheets.append(move(sheet));
 }
 
-StyleSheetList::StyleSheetList(Document& document)
+StyleSheetList::StyleSheetList(DOM::Document& document)
     : m_document(document)
 {
 }